Using the CONVERT command in RMAN can be an efficient way to migrate a database from one OS to another, if they have the same endian.
We migrated 9 databases on 10.2.0.4 from HP-UX (on Itanium) to AIX. The databases where distributed on three servers with not exactly the same hardware configuration. On one of the servers we encountered an error described in bug 888530 on Metalink or My Oracle Support. The process terminated prematurely with ORA-7445 when converting an undo tablespace. The other tablespaces were migrated fine, but the undo tablespace ended up with a missing datafile. After the error occurred the RMAN session was left hanging and after killing it the migrated datafile belonging to undo was removed; may be that had something to do with the file system being NFS mounted. Anyway, our solution was to create another undo tablespace and change the undo_tablespace parameter, and then dropping the corrupted one. In one of the tests we did, we managed to do recovery on the migrated undo tablespace, but some funny error messages showed up in the alert log, probably because it was not finally migrated to the new platform. Meaning, if you get this error, create a new tablespace for undo and drop the old one.